The authors have previously disclosed various inductive displacement detectors. Typically, the displacement of an inductive target is measured relative to a laminar antenna comprising transmit and receive windings arranged along the measurement axis. The inductive coupling between the transmit and receive windings varies according to the position of the target. In some embodiments, the antenna extends along the measurement path and in others, the target extends along the measurement path. These prior art embodiments offer reliable operation; accurate measurement; various detection geometries and low production cost. Low costs are achieved because the laminar constructions are suited to printed circuit board (PCB) production techniques.
High resolution or high accuracy detectors typically require repeating winding pitches along the measurement path. Typically, to avoid the problem of position ambiguity, a series of fine pitch windings co-operate with a single coarse winding. Alternatively, a Vernier arrangement of windings can be deployed using a first and second series of windings with different pitches so that there is no common multiple over the measurement path. A further alternative is the use of Gray code winding patterns.
The common approach is the use of coarse and fine pitch windings. In practice, the number of fine pitch windings per coarse winding is limited and for high accuracy applications it is usual for there to be coarse, intermediate and fine pitch windings. Such PCB constructions are complex because of the need for multiple windings, multiple pitches, multiple turns (for reasons of good inductive coupling), multiple PCB layers and many plated via holes. Such complex constructions are also problematic. Firstly, PCBs with many layers and plated through holes, are expensive to produce. Second, the via holes distort the magnetic field and hence degrade measurement performance. Third, there is an inherent tolerance in the registration of the windings on one layer with the windings of another which causes a variation in the measurement accuracy between components. Fourth, the choice of PCB substrate materials suitable for multi-layer PCB, plated via constructions is limited. Fifth, separations between windings on different layers complicates detector design because the separations require compensation. Sixth, the thickness tolerance on mainstream PCB laminations is large (typically 10% of nominal) which causes further variation between products and batches. Seventh, plated via holes suffer from thermal fatigue and cracking due to thermal cycling. Eighth, via holes can from undesirable traps for bugs or foreign matter in hygiene related applications. Ninth, in small detectors large numbers of via holes are difficult to arrange and take up a large amount of available space. Tenth, in small, fine pitch detectors small diameter vias (<0.5 mm) are necessary which are expensive and difficult to produce. Eleventh, the area required for multiple Verniers or multiple very fine, fine and coarse windings makes the detectors large and expensive. Twelfth, there is electromagnetic interaction between the different sets of windings (such as the coarse and the fine windings) which degrades measurement performance.